Until comparatively recently homogeneous metal catalysts have been used primarily in the conversion of unsaturated organic molecules, particularly those with olefinic and acetylenic bonds, to their hydrogenated, oxygenated, hydroformylated, and/or isomerized derivatives. Particularly favored were linear olefins both of the alpha and internal type. Homogeneous catalysts have become especially popular in these reactions because they generally offer several practical advantages over comparable heterogeneous catalysts. These advantages include one or more of the following, viz. faster rates of reaction, improved selectivities to desired product or products, resistance to common catalyst poisons such as sulfur compounds, and milder conditions of operation.
While solubilized catalysts have found special applications in the derivatization of olefins, far less has been published on their use in converting other functional groups not containing carbon-carbon multiple bonds. Recently, novel processes have been disclosed which enable homogeneous catalytic techniques to be applied to the selective hydrogenation of nitro(--NO.sub.2) compounds. Important applications for this technique include the reduction of nitroparaffin substrates to their paraffinic amines*, the selective hydrogenation of certain classes of mono and polynitroaromatics to their corresponding aromatic amines and nitroamines**, and the sequential hydrogenation of nitroaromatic mixtures, e.g., nitroaromatic-polynitroaromatic mixtures***. Not only are selectivities and yields good, but the resultant paraffinic or aromatic amines can be recovered from the crude reaction mix by standard experimental procedures used in amine isolation and purification. FNT *Knifton et al, U.S. Pat. No. 3,766,271 (1973) FNT **Knifton et al, U.S. Pat. No. 3,832,401 (1974) FNT ***Knifton et al, U.S. Pat. No. 3,903,167 (1975)
While in the case of nitroaromatic stocks, however, said processing is effective in hydrogenating broad classes of nitroaromatics, including mononitro-ortho-substituted polyalkyaromatics, such as 2-nitro-m-xylene**, it is not generally effective for the selective hydrogenation of other classes of ortho-substituted nitroaromatics that contain nonalkyl ortho-substituents. Likewise, many older catalytic hydrogenation techniques advanced in the open literature are unsuited for the selective reduction of ortho-substituted mononitroaromatics on account of cleavage of functional groups, cyclization and incomplete reduction. FNT **Knifton et al, U.S. Pat. No. 3,906,045 (1975)
Applicants disclose herein then a homogeneous hydrogenation technique employing solubilized ruthenium-containing complexes in combination with quaternary ammonium hydroxides to effect the selective hydrogenation of certain classes of nitroaromatic substrates containing bulky and/or reactive ortho-substituents, to their corresponding hindered aromatic amines. Alternative cleavage and cyclization reactions and incomplete reduction are not important with this class of catalyst, consequently selectivity to desired amine generally ranges from 85 to 100%.
By selectivity, as defined herein, is meant the efficiency in catalysing a desired conversion relative to other undesired reactions, in this instance reduction of nitro (NO.sub.2) groups on aromatic rings to the desired and corresponding amines. Selectivity is usually expressed as a percentage factor representing the amount of the amine formed, divided by the amount of starting nitroaromatic converted. Likewise, amine yield here refers to the fraction of amine formed, divided by the amount of nitroaromatic charged.
In view of the aforementioned deficiencies of the existing catalysts when applied to the hydrogenation of certain classes of hindered nitroaromatics, the catalysts of this invention represent a substantial advance in the art. Particularly, said catalysts satisfy in most instances the following objects:
(a) The ability to hydrogenate the nitro (NO.sub.2) function of nitroaromatics containing a variety of other functional groups including alkyl groups, halo, aryl, benzyl, acetyl, alkoxyl, halogenated alkyl, substituted amino, amino, carboxyl and alkylthio substituents.
(b) The ability to selectively hydrogenate nitroaromatic substrates containing bulky and/or reactive ortho-substituents wherein each NO.sub.2 -group to be hydrogenated is bonded via a C--N bond to an aromatic ring system, and one or both of the carbon atoms of the aromatic ring system adjacent to the carbon of the C--N bond are also bonded to bulky and/or reactive substituents (R and R' in eq 1) said substituents being selected from the functional groups including alkyl, halogenated alkyl, alkoxyl, acetyl, halo, amino and alkylated amino substituents, together with mixtures thereof. ##STR1## Other objects will become apparent to those skilled in the art after a perusal of this application.
In practice, the above objects, among others, will be achieved by the following procedure or minor modifications thereof.